Coevolution in model ecosystems with Escherichia coli and Coliphage lambda-virulent
The evolution of chemos tat E. aoZi-Avir communities was studied with the aim of clarifying the mechanism of their persistence. Under the physical and nutrient conditions of the experiments these organisms may coexist indefinitely but the pattern of population dynamics varies considerably from culture to culture. Generally, the evolution of phage-resistant bacteria stabilises the total bacterial numbers and reduces the density of the phage population. The latter persists, however, at very high densities and fluctuates irregularly. Two obvious evolutionary events, ·the evolution of resistance in bacteria and of a delayed-plaque-growth phenotype in phage, were associated with major changes in population densities. Through extensive phenotypic characterisation of the "selected" strains, however, many more evolutionary events were traced which did not correspond to density fluctuations in any obvious manner. The "selected" bacterial population consisted of several distinct maZ+- (ability to utilise maltose)'phenotypic subpopulations some of which were totally resistant to both the "selected" and the original phages; some were sensitive to the selected phage and to host-range mutants of, but resistant to, the original phage (partially resistant); some appeared resistant but acquired sensitivity in the presence of maltose (semi-resistant); some were fully sensitive to all phages. Other characteristics of the "selected" bacteria included mucoidity, larger cell size and longer intrinsic generation time, high spontaneous mutability, inhibition of phage intracellular development in the ,. presence of mitomycin C and low frequency of lysogenisation by temperate . strains, all suggestive of evolution of their SOS-repair, recombination, DNA metabolism and cell division gene-system. The altered antigenic specificity, the increased sensitivity to chloroform and the increased adsorbing ability of the "selected" phage are interpreted as pleiotropic effects of the evolution of its adsorption site. Its ability to infect partially resistant bacteria was, however, due to a second evolutionary event since the partially resistant isolates were found to block the infective process at a stage subsequent to the irreversible adsorption reaction. The evolution of the phage's adsorption site is explained a~ a response to the evolution of "semi-resistance". A third independent response of the phage was an extraordinary increase in its burst size which was expressed specifically on the genetic background of the coexisting selected hosts. An alternative to the existing model of the genetic Control of A-development is proposed, to explain the usual variation in burst ,size and in frequency of lysogenization as well as the evolution of these parameters; according to this model no "decision" is taken as to which developmental pathway will be followed. The delay of the selected phage in forming plaques on all hosts is suggested to be a side effect of the evolution of phage genes which control its development. The proposed model of A-development predicts that, 'although not isolated, lysogens and plasmid-carriers do exist in such "selected" communities and may, under certain circumstances, play important evolutionary roles. However, for as long as the extracellular conditions do not inactivate the virulent variants, the latter may persist by evolving their virulence (host range and productivity of infection) in response to any evolution of resistance, or immunity, in the coexisting bacteria. The extent and, mainly, the nature of evolution in these cultures, the conclusions of an extensive discussion of the E. aoZi-Avir systems biology, as well as the conclusions of a theoretical and empirical study of "fitness" and its determinants, severely contradict fundamental assumptions and specific (for this system) predictions of previous equilibrium and non-equilibrium theories of ecological stability. It is proposed that coexistence of E. coZi and 'Aviro is achieved through continuous, random, indefinite coevolution of resistance and virulence, respectively, and that the apparently erratic deviations of densities from constancy are the ecological - manifestations of the successive extinction and replacement of genotypic subpopulations.